Combustion systems in which a hydrocarbon fuel is oxidized to liberate energy in the form of heat have been studied extensively in order to optimize heat production and heat transfer. Those skilled in the art will appreciate that efficient energy utilization and pollution control are primary considerations in the design and operation of combustion devices. One conventional type of combustion burner includes a nozzle through which a fuel gas is flowed. At the nozzle discharge opening, the fuel and an oxidant gas, typically air, associate to form a combustible mixture. When this mixture is ignited, a self-sustaining oxidation reaction (combustion) is initiated which continues so long as the supply of reactants, the requisite ignition temperature and flame stability conditions are maintained. Since the fuel gas and the oxidant are not truly pre-mixed, this arrangement produces a diffusion flame in which oxidation occurs at the interface of the two reacting gases. Depending on the nature of the fuel and oxidant gases, various combustion products are produced. In those instances in which a fossil fuel is combusted in air, the hot, rapidly-expanding combustion gases predominantly comprise carbon dioxide, water vapor, and nitrogen, along with carbon monoxide, hydrogen, oxygen, hydrocarbons, nitric oxides and a host of other materials in trace amounts.
Since the primary object of most combustion systems is to provide a source of thermal energy, numerous configurations of combustion chambers and heat exchangers have been devised. As will be known to those skilled in the art, heat liberated by a combustion flame is transferred to the walls of a combustion chamber by two primary modes of heat transfer, convection and radiation. As the hot combustion gases move through the reaction apparatus, they contact heat transfer surfaces thereby directly transferring heat. In addition, heat is transferred by thermal radiation, a process which increases in significance as flame temperature and/or flame luminosity rises. As will be appreciated by those skilled in the art, radiative heat transfer occurs through both luminous and non-luminous processes. In luminous radiative heat transfer, bodies such as soot and other particles which are entrained or form in the flame radiate heat across a broad band of the infra-red spectrum, with a small fraction radiated in the visible part of the spectrum. In contrast, non-luminous radiative heat transfer comprises infrared emissions from hot combustion products, typically carbon dioxide and water vapor.
As will be explained more fully hereinafter, the present invention deals principally with the luminous component of radiative heat transfer. Fuels which have a high carbon-to-hydrogen ratio such as fuel oils and the like produce a substantial amount of soot during combustion. This production of soot significantly enhances radiative heat transfer. Conversely, those fuels having low carbon-to-hydrogen ratios such as methane which is the predominant constituent of natural gas, produce only minimal amounts of soot and thus have diminished luminous heat-radiative capacities. These fuels do, however, have higher non-luminous radiant output due to the increased production of water vapor during combustion. It has been determined that a general correlation exists between the luminous radiant output of a combustion flame and the carbon-to-hydrogen ratio of the fuel. As the carbon content of the fuel increases, the luminous radiant output of the flame increases due to an increase in soot production. In the present invention, this empirical relationship between carbon content of the fuel and luminous radiative heat transfer is circumvented.
The precise mechanism by which carbon particles form in diffusion flames is not well understood even though a significant amount of research has been conducted on this phenomenon. One theory holds that positively charged carbon ions are formed in a pyrolysis zone which serve as nuclei for carbon growth. Carbon particles may then associate to form larger aggregates of various sizes and shapes. It has been suggested by others that radiant heat transfer from flames can be controlled by regulating the residence time of naturally-formed carbon particles in the pyrolysis zone of diffusion acetylene flames with an electric field ("The Effect of DC 10 MHz Electric Field on Flame Luminosity and Carbon Formation", Eighteenth Symposium (International) on Combustion, the Combustion Institute (1981)). It will also be appreciated by those skilled in this art that although reduced carbon particle formation during the combustion of, for example, natural gas and other gases have low carbon-to-hydrogen ratios makes these fuels desirable from an environmental standpoint (lower particulate stack emissions), their reduced radiative capacity is a serious detriment. It would therefore be desirable to provide a means by which the heat radiative capacity of fuels having low carbon-to-hydrogen ratios could be enhanced, most preferably without increasing particulate stack emissions. It would be even more desirable to do so in a manner by which nitric oxide emissions could be reduced.
In order to increase the number of radiative bodies in a combustion flame, others in the past have preheated combustion fuel and/or oxidant gas. However, it has been determined that this technique requires temperatures in excess of 700 degrees C. for any significant particle formation to occur. The introduction of additives to a combustion flame has been attempted such as the addition of sulfur trioxide, chlorinated hydrocarbons, inorganic salts, titanium tetrachloride, fluorocarbons, and various other gases. For a number of reasons, these prior art methods have proven to be of little practical value. Most significantly, the cost of the flame additives increases the operational expense of the combustion system and often produces undesirable pollutants. The addition of carbon particles from an external source to the combustion flame has also been tried with limited success due to the inability to accurately control particle size. Moreover, triboelectric effects cause the carbon particles to agglomerate to sizes which cannot be fully burned in the combustion chamber and which do not attain high radiative temperatures due to the increased thermal inertia of large particles. These carbon agglomerates are then expelled in the exhaust gases. Particle agglomeration due to triboelectric charging has been observed even with submicron carbon particles. Furthermore, while the addition of carbon particles from an external source has provided some increase in radiant heat transfer, the resultant increase in stack emissions, the deposit of carbon on heat transfer surfaces (the latter operating to reduce heat transfer), and the relatively large particles which are produced are all serious detriments to conventional particle injection or precracking processes.
It is also known that hydrocarbon fuel gases such as natural gas and methane ca be decomposed pyrolytically in an electric arc of appropriate electrical characteristics. Gas plasmas formed by ionizing an inert gas such as argon have been studied extensively, primarily for their ability to generate extremely high temperatures which may exceed 10,000 degrees C. In these devices a gas or mixture of gases is flowed through an electric arc that is generated between a pair of spaced electrodes. Although the precise mechanism by which a gas plasma is formed is not fully understood, it is clear that gas molecules are ionized, forming positive and negative ions, and electrons which are accelerated toward the electrodes. The accelerated particles collide with other gas molecules which then also disassociate. In this manner, a plasma of ions, free radicals, and energetic electrons is formed. Although research has been conducted on the use of reactive gases to form gas plasmas, very little work has been done with natural gas.
One prior art electric arc apparatus which is particularly efficient when used to create a plasma of a gaseous feedstock is disclosed in U.S. Pat. No. 4,355,262 entitled, "ELECTRIC ARC APPARATUS," which is incorporated herein by reference. Therein, there is provided an apparatus for treating a material in an electric arc. The device includes a rod-shaped cathode having a flat end coaxially disposed in a converging throat portion of an annular anode. An annular gap is provided between the cathode and anode through which a stream of gas such as argon is flowed. An electric arc spanning the gap is initiated between the electrodes. Means such as magnetic field generating means is provided to produce movement of the arc along a predetermined path on the anode and the cathode. That is, the ends of the arc rotate about predetermined paths on the anode and the cathode. In this fashion, electrode erosion is significantly reduced.
In the above-mentioned patent disclosure the use of an electric arc device to generate active species from non-flammable gases for injection into a combustion apparatus is disclosed. It is postulated therein that pollution removal could be carried out, for example soot removal by oxidation or electric modification of the soot forming process by material injected from the electric arc device. It is also stated that this would increase the rate of oxidation of the soot and/or modify its electrical surface properties and hence aggregation characteristics, thus facilitating its removal from combustion products. It is further stated that combustion enhancement can be achieved in this manner by injecting hydrogen or nitrogen atoms to improve flame stability, provide ignition, and accelerate combustion in fuel-lean mixtures while decreasing pollution. This prior art device was developed to eliminate the need for argon in the gaseous stream. While the device is capable of generating an arc in a hydrocarbon gas without the use of argon, it was recognized by the present inventor that the device cannot operate for longer than two minutes due to the formation of a carbon pinnacle on the end of the cathode. As will be explained more fully herein, this carbon pinnacle, being at cathode potential, provides a migration path for the cathode spot, destabilizing the device. Moreover, the carbon pinnacle obstructs the anode orifice causing destabilization and extinguishment. The problem of carbon pinnacle formation is solved in the present invention. Also, it is disclosed that the prior art device could possibly be used intermittently on a hydrocarbon feedstock as an ignition source in a jet engine.
Therefore, it would be desirable to provide a combustion apparatus which utilizes an electric arc to generate carbon particles pyrolytically in a continuous flow of, for example, burning natural gas with the carbon particles serving as heat radiative bodies in a sustained combustion flame. Ideally, an apparatus of this nature would also provide a combustion flame which is stable over a wide range of fuel velocities and which would provide reduced nitric oxide emissions by radiative cooling of the flame produced by these heat-radiative bodies. It is to this end that the present invention is directed.